The present invention relates to a printing head drive apparatus for printing an image through use dots arranged in a staggered pattern by a thermal head or the like, and a method for driving a printing head.
For instance, in a thermal printer, an image is formed through use of a thermal head having heating elements placed in a line thereon. The thermal head forms an image on paper by controlling heating of each heating element corresponding to each dot. The outline of a printer equipped with a thermal head will be described by reference to FIG. 5.
A print data signal is input to the printer in a serial manner in accordance with a data transfer clock signal. The printer stores the thus input print data into a shift register 114. The instant when print data for one line are completely received, a latch circuit 115 latches the print data.
The latch circuit 115 actuates in unison switching elements 112 provided for respective heating elements 111, by outputting a strobe pulse signal in synchronism with a strobe synchronization signal. Consequently, the heating elements 111 whose switching elements 112 are brought into conduction receive power from a common electrode 125, thus becoming heated. By virtue of the heat, dots are formed on paper.
The conducting state (i.e., an ON/OFF state) of each switching element 112 changes in accordance with print data. A period of time during which the switching element remains in a conducting state also changes in accordance with print data for the dots (an area ratio of dots).
Recently, there has been employed a control technique whose principal purpose is to improve image quality and which involves forming pixel dots in a staggered pattern on paper by shifting timing at which a voltage is applied to the heating elements (i.e., heating timing). such a technique is described in, e.g., Japanese Patent Application Laid-open Hei. 7-312677.
Because of circuit configuration, heating is commenced in a synchronized manner for all the heating elements 111 of the existing printer (see FIG. 5) used for forming one line, and the timing at which a voltage is applied to the heating element cannot be uniquely controlled an each-heating-element basis (nor on a group-by-group basis, provided that the heating elements for one line are divided into a plurality of groups). Accordingly, to commence application of a voltage to a certain group of heating elements, application of voltage to another group of heating elements must be completed.
In order to divide the heating elements for one line into a plurality of groups and to apply a voltage to the groups at different timing, a time period capable of being assigned to one group (i.e., the length of time during which a voltage can be applied to the group) must be reduced to a time period shorter than that used for a printer which does not apply a voltage to the heating element at shifted timing.
Further, because of circuit configuration, print data must be input to the printer immediately before application of a voltage to the heating elements. Since solely the timing of heating of an element cannot be shifted without reference to transfer of print data, there arises dead time during which print data are transferred. Accordingly, in effect, the length of time during which a voltage can be applied (i.e., the width of a strobe pulse) is reduced to a greater extent.
FIG. 6 shows strobe pulse signals for black (K) and cyan (C) in a case where the timing of heating is controlled by dividing the heating elements into an odd-numbered group and an even-numbered group. As shown in FIG. 6, the length of time capable of being assigned to each group is reduced to a value (e.g., 3.5 ms in the example) less than one-half that used for the known printer (i.e., a printer in which all the heating elements used for forming one line are heated in a synchronized manner).
Such being the case, if dots are output at an area ratio of 100% (i.e., what is called a solidly shaded image is formed), clearance is created among the dots, resulting in a failure to reach an area ratio of 100%. For this reason, in a case where dots having a large area ratio are formed, the voltage applied to the heating elements by way of the common electrode is increased to a value greater than an ordinary voltage to thereby supply greater power (or energy) to the heating elements within a shorter period of time, thus compensating for creation of clearance.
However, an increase in voltage results in an excessive increase in the peak temperature of the head, and anomalies stemming from overheat sometimes arise in printing material.